In normal individuals the basal heart rate varies by approximately 20 beats per minute over 24 hours. A heart rate of 70-80 may be typical as a basal heart rate during waking hours; however, throughout most of the sleep cycle the heart rate is considerably slower. Although this circadian heart rate pattern has been known for many years, only recently has there been evidence that the nocturnal slowing of the heart rate may have an important restorative function. We previously studied 9 patients with dual chamber pacemakers and normal left ventricular function utilizing a crossover design. We found that these subjects had measurably better left ventricular stroke volume and systolic function after 3 weeks of pacing at 50 bpm than they had after 3 weeks of pacing at 80 bpm. None of the patients in our original study, all of whom had normal ventricular function, had any change in symptomatology at either pacing rate. Patients with impaired left ventricular function would be predicted to benefit from improvement in ventricular function. The purpose of this study is to evaluate pacemaker patients with less than normal ventricular fucntion to determine if they may benefit clinically or subclinically from slower nocturnal pacing rates. Nine subjects have completed the present protocol. Preliminary data analysis indicates that unlike the subjects with normal ventricular function who noticed no difference in symptoms at either pacing rate, the subjects with impaired LV function are more likely to have symptoms of fatigue and dyspnea when paced for 4 weeks at a rate of 75 bpm compared to pacing at a rate of 50 bpm. These data suggest that non-pacemaker dependent patients with impaired LV function may benefit from pacing at lower rates than may now be common practice.